1. Field of the Invention
Non-planar ceramic chip carriers, i.e., those containing raised regions or recessed cavities, are sometimes required for specific applications. Processes for producing packages containing cavities or raised areas involve numerous steps which add to the cost. These can include forming cavities in greensheets by processes such as laser machining or mechanical punching, and the subsequent lamination of the stacked sheets with inserts to maintain the shape of the cavities.
There are numerous tools and devices which must be maintained, and handled, to support these processes which are unique to non-planar substrates. Hard tooling such as inserts and mechanical punches are product-specific, are expensive and often have a long lead time for each new product design and material. The present invention relates to simple, flexible methods to produce non-planar substrates.
2. Description of Related Art
Multi-layer substrates for producing packages or carriers for electronic circuits can be produced from unfired ceramic tape in many configurations. The most often used process involves the production of a `greensheet` by casting a suspension of ceramic powders onto a metal or polymer film using a well known doctorblade process. These greensheets can be cut, punched, screened with pastes containing metal powders to form conductive patterns and subsequently laminated, usually at elevated temperature and pressure, to form a laminate. These laminated greensheets can then be heated (sintered) to produce a durable substrate for use in packaging electronic devices.
In some instances, the electronic device may need to be recessed below the outermost surface of the substrate. In this case, a recess or cavity can be formed in the substrate. This could allow, for instance, the use of wirebond interconnections within the cavity between the substrate and the integrated circuit. Since the greensheets are usually laminated under considerable pressure, there is a tendency for the layers within the cavity to distort unless they are restrained by an insert placed into the cavity before lamination. These inserts can be made from hard materials or have a hard surface to cause, for instance, wirebond traces to become embedded into the greensheet to produce a planar surface on the wirebond shelf after lamination.
Cavity inserts present several problems in a manufacturing environment. To reduce surface imperfections which can result from dirt and/or irregularities in the surface finish of the insert, the inserts can be made from hardened metals. These inserts may be lapped to produce an ultra smooth finish. The inserts are also made to extremely tight tolerances in all dimensions. Furthermore, each insert may need to be thoroughly cleaned prior to each use, which requires cleaning equipment and personnel and additional support processes. Cleaning also necessitates that many additional inserts be fabricated because of the turn-around time of the cleaning loop. Cavitied substrates also require other operations which include setting the insert into the stacked greensheets prior to lamination and removing them afterwards. This not only requires extra time, but may also subject the inserts to handling damage requiring their replacement. Difficulties may also arise when it is required to form complex cavities, such as cavities which extend through the substrate and/or those with more than two tiers, very small cavities and those with complex shapes.
The formation of cavities in the greensheets, prior to screening and lamination, can be accomplished by several mechanical means. Cavities can be formed by programmable tooling such as a CO2 laser or by mechanical step and repeat punching. These can be programmed to form any shape cavity but are generally too slow for volume production. Additionally, large amounts of debris can be formed by cavity formation processes requiring separate cleaning steps. This debris can contaminate screened features (eg. wirebond pads or seal bands) when they are screened prior to cavity formation and can cause defects in the substrate after sintering. A fixed punch and die set can also be designed and built to produce cavities but this usually requires a long turn-around time and is very expensive. Although it is capable of high throughputs, it is not practical for small to medium volume runs, and it may not be capable of forming cavities after screening. This is sometimes required when it is not possible to screen after forming cavities. In this case laser cavity formation may be the only solution. Mechanical cavity formation in unfired tapes or laminates is also not practical for very small or complex cavities.
It has been proposed by U.S. Pat. No. 5,240,671 to form cavities in ceramic layers by casting a ceramic slurry into a mold containing protrusions. This is not practical for a manufacturing process due to the handling and flexibility concerns, as well as the difficulty of having conductive metal embedded within the package.
It has also been proposed by U.S. Pat. No. 5,176,771 to form surface cavities in a laminated multi-layer ceramic substrate by the application of an outer surface material having a different thermal shrinkage rate than that of the remainder of the substrate, whereby delamination of said material occurs during the baking operations to leave a surface cavity in substrate.
Still other patents propose cavity formation produced by machining the fired substrate with abrasive tooling or processes. These are very slow and expensive.